1. Field of the Invention
The present invention relates to an electro-absorption optical modulator and, more particularly, to an electro-absorption optical modulator having a multiple quantum well, in which an absorption layer is formed by combination of quantum wells, each of which has a width different from each other.
2. Description of the Related Art
In digital optical communication systems, a semiconductor optical modulator used for signal modulation typically functions to regulate an intensity of incident light. In other words, an intensity of output light is controlled according to an input electrical signal. In this manner, digital signals, which are subjected to intensity modulation (IM), are simply differentiated as ones (state of “1”) having intensity higher than a predetermined reference level and ones (state of “0”) having intensity lower than the predetermined reference level.
This semiconductor optical modulator may be not only used to perform IM for digital communication, but also used as an analog optical modulator for controlling an intensity of output light according to an electrical signal having a predetermined frequency. The analog optical modulator has been utilized as the most important signal source in the optical transmission technology for an ROF (Radio-over-Fiber) link, in which an RF (Radio Frequency) signal, on which a signal undergoing digital modulation (e.g., Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), etc.) has been carried, is converted into an optical signal and then transmitted to an optical fiber. Meanwhile, it is most important for the semiconductor optical modulator to minimize modulation distortion of an optical signal for an input electrical signal. This functions to differentiate the analog optical modulator from the digital optical modulator.
Among RF optical modulators, an electro-absorption optical modulator having a multiple quantum well is a device having a high-frequency operating speed, low-power consumption and a capability to be integrated with other devices. For these reasons, the electro-absorption optical modulator attracts attention in the optical transmission technology for the ROF link. However, the electro-absorption optical modulator has a problem that, because of a non-linearity of an electro-optical transfer function, it is essential to overcome a phenomenon of signal distortion generated when signals are transmitted. The prior arts for overcoming these problems are as follows.
FIG. 1a shows a structure in which two electro-absorption optical modulators are integrated. In this structure, input optical signals are divided to have the same optical power using a 3-dB coupler 3. After each optical signal is allowed to be incident onto the optical modulators 1 and 2, a bias voltage and a modulation depth are appropriately selected to minimize intermodulation distortion (IMD) caused by non-linearity of a transfer function. That is, by decreasing a value of the IMD, linearity is enhanced.
FIG. 1b is a graph of distortion coefficients of transfer functions of two electro-absorption optical modulators. In the case that, when bias voltages to be applied to the optical modulators are selected, two bias voltages Vb1 and Vb2 at which a third harmonic distortion value becomes zero (0) are selected, a second order distortion coefficient (line A) has a similar magnitude but an opposite sign at the two bias voltages. Further, a third order distortion coefficient (line B) has a sign changed oppositely around the two bias voltages. That is, the second and third order distortion values can be minimized, and thus linearities of the optical modulators are increased [see M. Shin and S. Hong, “A Novel Linearization Method of Multiple Quantum Well (MQW) Electro-absorption Analog Modulator”, Jpn. J. Appl. Phys. Vol. 38 (1999), pp. 2569–2572].
FIG. 2 shows a structure in which two different wavelengths of light sources are incident onto an electro-absorption optical modulator. According to this structure, light having two carrier frequencies ω1 and ω2 are input to enhance linearity, respectively. A wavelength-tunable laser, which is capable of selecting light having a wavelength of 1.32 μm (frequency: ω1) as a primary light source 21 and light having a wavelength between 1.28 and 1.35 μm (frequency: ω2) as a secondary light source 22, is used. Two kinds of light emitted from the first and secondary light sources 21 and 22 are simultaneously incident onto an optical modulator 23. Here, an RF signal is input into the optical modulator 23 together with a bias voltage. In the case of frequencies of light output through the optical modulator 23, as shown in FIG. 2, each of the carrier frequencies ω1 and ω2 is located at the middle position, the RF signal are carried beside each carrier frequency, and synthetic frequencies ω1+3 ωRF and ω2+3 ωRF are located next to the RF signal. When a wavelength of the wavelength-tunable laser is properly adjusted, phases of unwanted frequency terms become 180°. Thus, in the case of optical signals output through a final photodetector 24 such as a high speed photodetector, only a frequency of ωRF is detected, so that the linearity is enhanced [see K. K. Loi, J. H. Hodiak, X. B. Mei, C. W. Tu and W. S. C. Chang, “Linearization of 1.3-μm MQW Electroabsorption Modulators Using an All-Optical Frequency-Insensitive Technique”, IEEE photon. Technol. Lett., Vol. 10, No. 7, July 1998, pp. 964–966].
FIG. 3 shows a structure in which predistortion is provided to an electrical signal applied to an electro-absorption optical modulator. According to this structure, linearity of the optical modulator is enhanced by providing predistortion to the electrical signal carried on the optical modulator. Prior to electrical distortion, a bias voltage of the optical modulator is selected at a point where a second harmonic distortion becomes zero (0), that is at an inflection point of the transfer function. This approach is allowed to leave even-order harmonic terms except a second order term and odd-order harmonic terms. The even-order harmonic terms and odd-order harmonic terms have an influence on the linearity of the optical modulator. In reality, as the order of the harmonic terms becomes higher, their magnitudes are drastically decreased. Thus, with regard to the harmonic distortion, second and third order terms have only to be taken into consideration. However, because the bias voltage is set to have the second order term become zero (0), the electrical signal is distorted so that the third order term may become zero (0). Referring to FIG. 3, an input signal Vin is split into two signals, one of which travels along a delay line, and the other is distorted into a signal having odd-order non-linearity by means of a non-linearity generator 31. When combined and input into an optical modulator 32, these signals function to allow odd-order harmonic distortion terms of the transfer function to become minimum, and thus the linearity of the optical modulator 32 is enhanced [see Gordon C. Wilson, Thomas H. Wood, M. Gans, J. L. Zyskind, J. W. Sulhoff, J. E. Johnson, T. Tanbun-Ek, and Paul A. Morton, “Predistortion of Electro-absorption Modulators for Analog CATV Systems at 1.55 μm”, J. Lightwave Technol. vol. 15, No. 9, September 1997, pp. 1654–1662].
As mentioned above, up to now, in order to enhance the linearity of the optical modulator, an external light source or an electrical circuit is additionally constructed, which results in a complicated and swelled system. In particular, in the case of a system like an ROF (Radio-on-Fiber) system using a super high frequency as the carrier frequency, because a travel distance of a wave is short, many base stations are required. Therefore, development of technologies capable of enhancing the linearity of the optical modulator in a simpler manner than the prior arts as exemplified above is required.